When an overvoltage in excess of a predetermined value is applied to a power transformer in an electric power system continuously for more than a predetermined period of time, it is necessary to protect the transformer from such a voltage by interrupting the same, and this requisite function is achievable by the use of a protective relay device. However, such an overvoltage is generated due to a variety of reasons including faults that occur in the transformer itself and some other components in the electric power system; the relay device is thus required to have arrangement not to respond to any of mere transient overvoltages other than those resulting from the faults.
FIG. 1 is a block diagram of a conventional protective relay device, wherein an input voltage Vin applied to a power transformer (not shown) to be protected is introduced to a rectifying-smoothing circuit A1, so that the voltage Vin is rectified and smoothed to a DC voltage signal and then is applied to a DC level detector A2. The detector A2 compares the output voltage signal from the circuit A1 with a reference voltage Vref generated from a reference voltage generator A3, which is selected in accordance with the overvoltage to the power transformer. If the former voltage is higher, the detector A2 provides an overvoltage detection signal Dov to a timer A4.
The timer A4 is equipped with a time count function which is correspondingly set to a time period T of the duration of a transient voltage generated usually by a rush current in a transformer or on/off action of a switch in an electric power system, so that it produces a signal TRIP so long as the duration of the signal Dov exceeds the time period T. The signal TRIP serves to actuate an circuit breaker (not shown) for disconnecting the transformer from the overvoltage.
FIG. 2 graphically represents the response characteristic of the protective relay device shown in FIG. 1, wherein an input voltage Vin is plotted along the abscissa and a time t (ms) required for response is plotted along the ordinate. Lines AT and CT denote the operation time and the rest time of the relay device respectively, and a line TV denotes a threshold voltage-and-time product permissible for that power transformer. As is obvious from FIG. 2, the line AT has a considerable deviation from the line CT in achieving reliable protection of the transformer. With the detector A2 responsible to such a single reference voltage level, it is difficult to provide satisfactory response characteristic for the device.
An object of the present invention is to provide an overvoltage protective relay device which is capable of performing a highly reliable operation for protection of a power apparatus.
And another object of the invention is to provide an overvoltage protective relay device responsive accurately to any overvoltage in accordance with the characteristic curve of the voltage-and-time product permissible for a subjective power apparatus which is to be protected from the overvoltage.